Modification of H-mode pedestal structure with lower hybrid waves on Alcator C-Mod

Abstract

The application of lower hybrid waves (LH) in H-mode plasmas on Alcator C-Mod can result in significant reduction of core particle inventory, with no significant degradation of energy confinement. This phenomenon has been observed in steady enhanced D α (EDA) H-mode targets, which are sustained by ion cyclotron RF auxiliary heating, in which pedestal density n ped is usually tied firmly to plasma current I P and shows a strong resilience to changes in the edge neutral source. Upon application of up to 1MW LH power, n ped is reduced by up to 30%, while the temperature profile increases simultaneously such that the pressure pedestal remains constant or is slightly increased.Steady EDA H-mode operation with no edge-localized modes (ELMs) can be maintained while edge collisionality is reduced by factors of reductions of 2-4. Elevation of scrapeoff layer (SOL) density and electric currents accompany the application of LH (at levels as low as 400kW) with a fast time response (~10 -2 s), while full density pedestal relaxation and core density reduction occur on longer time scales (~10 -1 s). A similarly prompt counter-I P change in the edge toroidal velocity is also observed in response to LHRF, followed on longer time scales by a counter-I P change in the central rotation. The range of time scales of the plasma response may indicate that the radial locations of LH interactions (i.e., SOL vs. core), and power deposition mechanisms, are evolving in time.Understanding the responsible physical mechanisms and applying them to a broad range of H-mode discharges could provide a tool for improving density control, affecting edge MHD stability and applying modulation for transport studies.

Effects of LHRF on toroidal rotation in Alcator C-Mod plasmas

“…While there was an increase in the core electron temperature, there was no peaking of the temperature profile, and no change in the stored energy or energy confinement time. This peaking of the density profile (fourth frame of figure 20) is the result of an edge density pedestal reduction [23]. A comparison between electron density and toroidal rotation profiles during the ICRF H-mode phase, and with additional LHRF power, is shown in figure 21.…”

“…The measurements were made just below the outboard midplane and map to a location ∼8 mm inside the LCFS. in the absence of a co-current change in edge toroidal rotation [23]. Reasons for these differences are not clear, although there may be significant differences in the quality of LH wave absorption.…”

“…One possible difference is that the discharge without the density peaking and rotation drop had a substantially higher radiated power (by about 25%) at the time of the LHRF pulse, perhaps because this plasma was later in the day, after the boronization wall conditioning had started to wear off. It has been shown [23] that excess core radiation prevents LH-induced modification of the pedestal. The correlation between the counter-current increment in the core rotation (indicative of a flattening of the velocity profile) and the peaking of the electron density profile is demonstrated in figure 23.…”

Lower hybrid wave edge power loss quantification on the Alcator C-Mod tokamak

“…19 Changes in H-mode quality, in the pedestal, and in the edge fluctuations were observed due to LH wave power in regimes with negligible current drive. 20,21 Several separate observed phenomena highlight the importance of the LH wave power in the edge plasma at high density.…”

Physics news on the Internet (based on electronic preprints)

“…At excessively high densities, however, the LH wave can be absorbed in the SOL instead of the main plasma. In the Alcator C-Mod tokamak, LH waves routinely affected the H-mode pedestal, leading to reduced density and particle confinement at constant energy confinement [172], i.e. the temperature increased to compensate for the density pumpout.…”